Method and device for bonding substrates

ABSTRACT

A method and corresponding device for bonding a first contact surface of a first substrate to a second contact surface of a second substrate. The method includes the steps of arranging a substrate stack, formed from the first substrate and the second substrate and aligned on the contact surfaces, between a first heating surface of a first heating system and a second heating surface of a second heating system.

FIELD OF INVENTION

This invention relates to a method for bonding a first contact surfaceof a first substrate with a second contact surface of a second substratewell as a corresponding device.

BACKGROUND OF THE INVENTION

Alignment systems are known in which the alignment of the substrateswith one another takes place under a normal atmosphere. The substratesthat are aligned with one another are still temporarily attached to oneanother in the alignment system and are subsequently transferred to ahigh-vacuum bonding system and permanently bonded there under vacuum,whereby the temperature that is necessary for bonding is produced byheat sources.

The heat sources together with corresponding pressure plates are locatedabove or below the substrates that are to be bonded. The upper heatsource and the upper pressure plate are designed to be movable. Thelower heat source and the specimen holder on the bottom of thehigh-vacuum bonding system are designed statically.

In the alignment system, the two substrates that are to be aligned withone another are attached to a bond chuck. In this case, the attachmentis carried out in most cases with mechanical clamps. The substrates thatare received in the bond chuck and are aligned with one another are thentransported to the high-vacuum bonding system. The bond chuck is laiddown on the lower heat source.

Because of the structures that are becoming steadily smaller,deformations of the substrates that occur because of thermal action havea negative effect on the bonding result.

SUMMARY OF THE INVENTION

It is therefore the object of this invention to provide a method withwhich the bonding result is improved.

This object is achieved with the features of the independent claim(s).Advantageous further developments of the invention are indicated in thesubclaims. Also, all combinations that are comprised of at least two ofthe features indicated in the specification, the claims and/or thefigures fall within the scope of the invention. In the indicated rangesof values, values lying within the above-mentioned limits are also to bedisclosed as boundary values and can be claimed in any combination.

The basic idea of this invention is to heat the substrates of thesubstrate stack that is aligned at the contact surfaces of thesubstrates, in particular, for the permanent bonding in the bondingsystem gently and uniformly from two surfaces to be heated (the sidesfacing away from the contact surfaces) of the substrate in order tominimize or to completely avoid in particular different deformations ofthe two substrates.

The invention describes in particular a modularized device as well as aprocess chain, which makes it possible to implement an alignment systemfor alignment and for temporary connection of two or more substrates toa substrate stack. In this case, the substrates that are aligned hereinare transferred to a bonding system taking into consideration anespecially sensitive path over a high-vacuum transfer section (i.e.,without interruption of the vacuum). In the latter, the substrates arejoined (bonded) durably and inseparably (i.e., permanently) withoutexposing the substrates to excessive or different expansions anddeformations.

The alignment system is preferably designed in such a way that forattaching the substrate stack in the alignment unit before the transferinto a bonding unit, connecting elements that do not contaminate thesubstrate stack and that temporarily hold the substrate stack togetherare provided. The connecting elements can be removed without residueafter the bonding.

In one embodiment of the invention, the substrate stack is processedwithin a vacuum transfer section to improve the bonding result with oneor more systems for reduction of surface gases and/or moisture.

In particular, the invention deals with a unit or device and a methodwith which the substrate stack is or can be symmetrically heated. Thecontact surfaces of the substrates of the substrate stack that arebrought into contact are used in particular as planes of symmetry. Inother words, the substrate stack is heated equithermally from bothsides.

The invention accordingly deals in particular with a method and a devicefor heating and bonding a substrate stack. The heating is preferablydone symmetrically, in particular by infrared radiation.

According to another aspect of this invention, the components accordingto the invention are integral parts of a unit, in particular ahigh-vacuum cluster, which is designed in particular in modularizedform. In the high-vacuum cluster, a process chain can be implementeduntil a substrate stack is bonded. In the high-vacuum cluster, specialattention is paid to the sensitive handling with the substrates that areto be bonded. The sensitive handling relates in particular to a uniform,in particular symmetrical, introduction of heat into the substrates tobe treated by different heating zones, which preferably is achieved byan upper dynamically movable heating system and associated chucks, and alower, in particular rigid or static, heating system.

An especially gentle bonding process is preferably made possible bybringing the upper heating system close against the lower heating systemin a controlled manner.

The process steps according to the invention are preferably run whilepreserving/maintaining the vacuum produced in the alignment unit, inparticular also during the transport by the vacuum transfer section witha vacuum transfer chamber, in particular a high-vacuum transfer chamber.

In a special embodiment of the device, in particular a high-vacuumbonding device, it is conceivable, in addition to a symmetricalintroduction of heat, preferably by uniformly bringing the oppositeheating surfaces of the heating systems close to the substrates to beprocessed, to control the substrates based on their type of material aswell as their thickness and/or to control the introduction of heat withconsideration of the expansion coefficient in order to avoid differentdeformations, in particular expansions and/or distortions, in particularalso when using different types of materials. In particular, thesymmetrical heating serves to ensure the uniform (i.e., homogeneous)through heating of both sides of the substrate stack.

Important process parameters are in particular the heating rate and/orthe heat output. The heating rate defines to what degree the substrateis heated per unit of time. In particular, the heating rate is set at avalue of between 1° C./minute and 1,000° C./minute. Lower heating rateswould be possible, but they are no longer economical at the targettemperatures since the heating process would take too much time. Heatingrates that are higher than 1,000° C./minute would quickly heat thesubstrates in such a way that damage to the substrates and/or alignmenterrors between the substrates can occur. The heat output indicates theamount of heat per unit of time, which is required in order to heat anobject to a preset temperature. The heat output is applied by heatingelements arranged in particular in the bond chucks, which arepredominantly operated electrically. Under the assumption of idealconditions, and therefore no power losses at all and a 100% conversionof the introduced electrical energy into heat energy, a lower limit ofthe necessary (electrical) output can be calculated, which is at leastnecessary to bring a substrate with a specific diameter and a specificthickness to the desired temperature within a desired time. The heatoutput corresponds to the energy per time unit that is introduced into asubstrate. The energy that is introduced corresponds in particular tothe heat that is introduced. The heat that is necessary to heat a bodyfrom a starting temperature to an end temperature corresponds to theproduct from the temperature difference that is to be overcome, specificheating capacity and weight of the body. The weight of the body can becalculated at a given density and dimensions. Substrates are preferablyround and very often single crystals. Therefore, they have geometriesthat are to be calculated and a homogeneous density. The calculation ofthe weight as a function of the geometric parameters, in particular thethickness and the diameter, can be done in an automated manner accordingto the invention. The thus determined amount of heat is then divided bythe desired minimal time in which the corresponding temperaturedifference is to be overcome. This leads to the heat output. Theelectrical power is then correspondingly selected. Since the entireelectrical power is not converted into heat output, the actualelectrical power that is to be introduced is selected correspondinglyhigher in order to compensate for power losses.

Material Parameters C₅ Process Parameters J kg⁻¹ Density Density t d VMass ΔT Q t L K⁻¹ g cm⁻³ kg m⁻³ m m m³ kg K J Minutes Watts 710 2.3362336.000 0.000725 0.200 2.27765E−05 0.053 475.000 17943.72795 1 299.062710 2.336 2336.000 0.000725 0.200 2.27765E−05 0.053 475.000 17943.727955 59.812 710 2.336 2336.000 0.000725 0.200 2.27765E−05 0.053 475.00017943.72795 10 29.906 710 2.336 2336.000 0.000725 0.200 2.27765E−050.053 475.000 17943.72795 15 19.937 710 2.336 2336.000 0.000725 0.2002.27765E−05 0.053 475.000 17943.72795 20 14.953 710 2.336 2336.0000.000725 0.200 2.27765E−05 0.053 475.000 17943.72795 25 11.962 710 2.3362336.000 0.000725 0.200 2.27765E−05 0.053 475.000 17943.72795 30 9.969710 2.336 2336.000 0.000725 0.200 2.27765E−05 0.053 475.000 17943.7279535 8.545 710 2.336 2336.000 0.000725 0.200 2.27765E−05 0.053 475.00017943.72795 40 7.477 710 2.336 2336.000 0.000725 0.200 2.27765E−05 0.053475.000 17943.72795 45 6.646 710 2.336 2336.000 0.000725 0.2002.27765E−05 0.053 475.000 17943.72795 50 5.981 710 2.336 2336.0000.000725 0.200 2.27765E−05 0.053 475.000 17943.72795 55 5.437 710 2.3362336.000 0.000725 0.200 2.27765E−05 0.053 475.000 17943.72795 60 4.984

The table shows the minimum electrical power that is necessary to heat asilicon substrate with a diameter of 200 mm and a thickness of 725 μm inthe indicated times from 25° C. to 525° C. The actual electrical poweris to be higher based on power losses. The electrical power of theheaters used is in particular higher than 100 W, preferably higher than1,000 watts, still more preferably higher than 5,000 watts, mostpreferably higher than 10,000 watts, and all the more preferably higherthan 20,000 watts.

According to the invention, the pressurization of the first substrate iscarried out in particular by a first pressure surface of a firstpressure plate, which preferably is the first heating surface at thesame time. The pressurization of the second substrate is carried out inparticular by a second pressure surface of a second pressure plate,which preferably is the second heating surface at the same time.

According to the invention, in this respect, in particular formulas orprocess sequences that are to be determined and secured in advance areused, which preferably take into consideration material-specificproperties and/or the specific conditions, in particular the materialstrength, of the substrates that are to be bonded. In addition to apossible, fully automatic processing of the formulas, a semi-automaticuse of the system as well as, moreover, a manual use of the system inquestion is also conceivable.

If the substrates to be bonded are comprised of materials that verystrongly reflect the thermal radiation and thus considerably slow downor impede a desired introduction of heat, the formulas are matchedaccordingly. Conversely, however, an increased absorption of the thermalradiation by correspondingly highly-absorbent materials and/or surfaceconditions would also be conceivable, whereby the formulas, inparticular heating times, would be correspondingly shortened.

The system according to the invention is in particular able to detectfactors that influence the heating, in particular by sensors, and totake these factors into consideration in the process sequence and/or toadjust these factors in the case of subsequent steps or subsequentsubstrate stacks.

In addition, the system according to the invention makes it possible ina preferred way to treat the substrates by convection and/or variousheating zones advantageously so that a uniform introduction of heat isachieved. The heating of the substrates is preferably done viaconvection of a gas that is introduced into the bonding system, inparticular into the bond chuck, still more preferably a studded chuck(disclosed in particular as an independent aspect of the invention).While the gas provides for the transfer of heat by convection, thebonding system is preferably continuously evacuated, so that in thebonding chamber, a pressure of less than 10⁻² mbar, still morepreferably less than 10⁻³ mbar, is set. The introduction of the gas isdone in particular before and/or during an evacuation of the bondingsystem. The gas can be directed from outside, preferably by a line ofthe bond chuck and/or a pressure plate with respect to the substratestack and/or bond chuck or pressure plate. In the case of a useaccording to the invention of a bond chuck with a studded contactsurface, in particular heating surface of the heating system, the gas isoptimally distributed between the substrate holder and/or the pressureplate and the substrate stack based on the defined topography.

While the substrate stacks that are aligned with one another lie in thebonder (in particular on a studded chuck) and the surrounding areaaround the substrates is evacuated, a gas that ensures the conduction ofheat between the (studded) bond chuck and the substrate stack flows (inparticular from the studded chuck). Evacuation is done around theoutside, and flushing is done between the substrate stack and the(studded) bond chuck. By flushing, the pressure applied by the vacuumsystem is correspondingly reduced in the bonding chamber.

One of the most important aspects according to the invention ispreventing a shifting of the substrates toward one another from beingcarried out by the heating of substrates, by which the alignment of thesubstrates with one another is lost again. Viewed in physical terms, theinvention ensures that the thermal expansions of the two substrates thatoccur in the case of a temperature change are always the same. If bothsubstrates are comprised of the same material, the two substrates thusare preferably equally tempered.

In further development of the invention, the heating systems aretherefore controlled during heating and converging in such a way that atleast during the vast majority of the converging period, the differencebetween the mean temperature of the first substrate and that of thesecond substrate is smaller than 5° C., in particular smaller than 1°C., preferably smaller than 0.5° C., and still more preferably smallerthan 0.1° C. Should the two substrates that are aligned with one anotherbe comprised specifically of the same material but have differentthicknesses, it can happen that the temperature change in one of the twosubstrates, in particular in the substrate with the larger thickness,runs slower than in the other substrate, in particular the one with thesmaller thickness. In such a situation, a shifting of the two substratescan therefore occur during the heating process despite identicalmaterial characteristics, but different geometry characteristics. In theideal case, however, the alignment produced in advance by an alignmentunit would be correctly adjusted again after the total heating of thesubstrates. Should the materials and thus the material characteristicsof the two substrates, in particular the thermal expansion coefficientor the thermal expansion tensor and/or the heat conductivity and/or theheat capacity be different, the result is an undesirable shifting of thesubstrates toward one another. This can be compensated by methodsaccording to the invention. In this case, the heating systems arecontrolled during heating and converging in such a way that at leastduring the vast majority of the converging period, the differencebetween the mean temperature of the first substrate and that of thesecond substrate is greater than 0.1° C., in particular greater than0.5° C., preferably greater than 1° C., and still more preferablygreater than 5° C. to compensate for the difference between the thermalexpansions of the two substrates, so that the alignment is maintained aswell as possible.

As an alternative or in addition, it may be advantageous to control theheating systems during heating and approaching in such a way that theradiation energy of the first heating surface that strikes the firstsurface during approaching is the same as the radiation energy of thesecond heating surface that strikes the second surface.

According to another advantageous embodiment of the invention, animprovement of the bonding result is achieved by a heating of thesubstrates upstream from the bonding and thus accompanying a gasemission, in particular within the vacuum transfer section and evenbefore reaching the actual bonding system.

After reaching the bonding system and even before the actual bondingprocess, a uniform or homogeneous (symmetrical or asymmetrical) heatingof the substrates, adapted to the individual requirements, is preferablyperformed. In this respect, (thermal) stresses owing to unevenexpansions within the substrates are avoided to a large extent, whichhas a positive effect on the desired bonding result. The thermalexpansion coefficient of silicon is, for example, in the range of 2.610⁻⁶ K⁻¹. If both substrates are manufactured from the material silicon,the two also have the same (averaged) thermal expansion coefficients(under the assumption that both substrates have the same crystalorientation, whereof in most cases, it can be assumed as most siliconwafers have a crystallographic (100) orientation, and under theassumption that the thermal expansion in the substrate plane isisotropic). Accordingly, substrates that are equally loaded thermallywill also experience the same thermal expansions. Should the materialsbe different, their thermal expansion coefficients are in generaldifferent. These different thermal expansions can be compensatedaccording to the invention.

According to another, preferred aspect of this invention, a receivingsystem for receiving the substrate stack at least during the bonding isprovided. The receiving system is suitable in particular for receivingthe substrate stack, in particular on the periphery or in a peripheralarea. More preferably, the receiving system is suitable to hold togetherthe substrates on the contact surfaces and to hold them in the alignedposition until the substrates are bonded. The receiving system, inparticular the studded bond chuck, preferably presses or brings intocontact only a small section of the surfaces of the substrates facingaway from the contact surfaces; the receiving system preferably coversthe surface of the substrates, to be loaded with radiation heat, barelyor not at all. In other words, the majority, in particular at least ¾,preferably at least 90%, of the substrate surfaces exposes the heatingsurfaces of the heating systems arranged at some distance away.

Due to a reduction of the surface of the substrates, which is broughtinto contact by the receiving system, an undesirable contamination ofthe substrates is reduced. In addition, with the described procedure,the throughput is increased, since the complete heating-up andcooling-down of the receiving system predominantly covering at least onesurface that was previously time-intensive and necessary after eachbonding process can be eliminated.

Another aspect of the embodiment according to the invention is that aheating of the substrate stack is done on both sides at least during theheating up to the bonding temperature at least predominantly by heatradiation and/or heat convection, in particular by natural heatconvection. This is produced in particular by spacing the substratesurfaces from the heating surfaces.

By the distance during the heating being reduced, the heat outputintroduced by heat radiation increases relative to the heat outputintroduced by heat convection. As soon as the heating surfaces,preferably simultaneously, bring into contact the respective surface ofthe substrates, the heat output is transferred at least partially,preferably predominantly, by heat conduction between the surfaces andthe heating surfaces. Preferably, at least up to ¾ of the bondingtemperature is reached before heat conduction and/or pressurizationis/are carried out.

A possible heat convection from above and below has the advantage thatexisting irregularities that impede a uniform and advantageous heatingof the substrates can be greatly reduced or completely precluded duringthe bonding process.

In a special, in particular separate according to the invention,implementation of the bond chuck and/or the heating surfaces, theycomprise studs to ensure the lowest possible contamination of thesubstrate surfaces. This also makes it possible in addition to supplythe gas necessary for convection during pressurization.

The contact surfaces of the studs can be designed in particular in abomb-shaped manner in order to further reduce the contact surfaces ofthe heating surfaces with the substrate surfaces.

The studded surface is further developed in a preferred embodiment bythe entire radial peripheral area of the heating surface orpressurization surface being bounded by a sealing ring. In thisconnection, an escape of the gas directed into the studded pressureplate is prevented or reduced. The peripheral areas of the upper andlower studded pressure plates in particular seal off in a positivemanner with the substrate to be processed and thus prevent the gas fromescaping.

According to the invention, it is conceivable to provide a passage thatadvantageously extends between the arm that bounds the entire radialside area and the substrate that is to be processed and over which it ispossible to discharge excess gas from the studded surface, in particularin a controlled manner, preferably in the form of a valve.

Process-accelerating measures, such as the contamination-reducingautomated transfer of the substrates aligned in the alignment unit by aprocess robot located within a vacuum transfer section with anespecially distinct receiving system, in particular in the form of asupport surface, which provides a contact of the substrate only on theradial periphery, are not yet known. This receiving system can,moreover, in addition be exposed to an electrostatic force in order tofurther increase the receiving forces of the process robot.

Also, the alignment of upstream mod/or downstream processes such as thetempering of the substrates can be viewed as an advantageous embodimentof the invention in order to remove surface gases and/or moisture beforeintroduction into the vacuum bonding system and thus to improve thebonding result.

As another, in particular separate, invention, a continuous treatmentfrom the alignment up to bonding in a vacuum, in particular a highvacuum, that continues through all, in particular modularized,components, is disclosed, in which both foreign atoms and reactions ofsubstrate material, in particular oxidation, are ruled out to a verylarge extent.

Another advantage of the embodiments according to the invention is theincrease in precision of a fabricated bond. Below, it is understood thatthe positioning accuracy of several structures of the substrates withrespect to one another, achieved by the alignment unit, is not lost bythe heat treatment in the bonder. In the state of the art, deformationsoccur in particular in that inhomogeneously heated substrate stacks, forexample based on substrates that are comprised of different materialsand thus different thermal expansion coefficients, shift or are deformedbefore the bonding process is concluded, i.e., the process of thepressurization begins or takes place. The invention allows, however, thepreservation of the adjustment accuracy of the alignment system betweenseveral structures up to and beyond the actual bonding process. Theachieved and retained alignment accuracy, i.e., the maximum shiftingbetween the structures at the opposite substrates of the alignedsubstrate stack until the completion of the bonding, is, according tothe invention, in particular less than 10 μm, preferably less than 1 μm,more preferably less than 100 nm, and most preferably less than 10 nm.

In isothermal operation, the embodiment according to the inventionallows an increase in the throughput since the heaters and thus the bondchucks as well as the pressure plates preferably are not constantlyheated and cooled, but rather can produce, in particular at leastpredominantly, preferably exclusively, by a change in positioning, atemperature change on the substrate stack. Components of the bonder, inparticular at least one of the pressure plates, can be moved almost atfull bonding temperature into a stand-by position and can wait there forthe next substrate stack, without having to be cooled.

The contamination of the substrates of the substrate stack is lower,since the substrate stack does not lie predominantly on the surfaces tobe heated until there is pressurization but rather is held in abeyanceand thus has virtually no contact with contamination material. In thecase of contact for pressurization, according to the invention, astudded bond chuck is preferably used in particular also as a pressureplate and/or heating surface, which minimizes the effective contactsurface to the substrates even in the case of the pressurization. Thecontact surface of the studded bond chuck according to the invention isin particular smaller than 90%, preferably smaller than 50%, morepreferably smaller than 25%, most preferably smaller than 5%, and withutmost preference smaller than 1%, of the substrate surface.

The device according to the invention is comprised in particular of amodular designed system/unit, which in a first embodiment comprises atleast three modules to be controlled independently of one another,namely an alignment unit (i.e., aligner), a vacuum transfer section withan integrated handling system, in particular the process robot forhandling substrates, as well as a bonding unit. For processing reasons,supplementing with additional modules is possible. In particular, manymodules can be provided in multiple ways for acceleration and parallelprocessing.

The charging of the alignment unit with the at least two substrates tobe aligned is carried out manually, but can preferably be donesemi-automatically or even more preferably in a fully-automated manner.

The substrates are preferably wafers. The wafers are standardizedsemiconductor substrates with well-defined, standardized diameters. Thesubstrates can have any shape. In particular, the substrates can berectangular or round. Should the substrates be round, the diameters ofthe substrates can also be of any size but in most cases arestandardized diameters of 1, 2, 3, or 4 inches as well as 125, 150, 200,300 or 450 mm.

Hereinafter, in the patent specification, substrates are referred to ingeneral. In particular, the embodiments according to the inventionrelate primarily to wafers, however.

The substrates are aligned with one another and form a substrate stack.The substrate stack is comprised of at least two substrates. However,more than two, preferably more than five, more preferably more than ten,and most preferably more than 15 substrates can also be aligned with oneanother and can be connected temporarily to a substrate stack. Thesubstrates can be comprised of any material. Preferably, materials thatare used in the semiconductor industry are involved. They preferablyinclude semiconductors such as silicon, germanium, glasses, such as, forexample, quartz, semiconductor heterostructures such as GaAs orceramics. Also, even the use of polymer substrates or metal substrateswould be conceivable. The thicknesses of substrates vary between 10,000μm and 50 μm, whereby substrates with correspondingly small thicknessesare produced in the respective thickness by grinding and polishingprocesses. Carrier substrates that are used only for supporting othersubstrates, the so-called product substrates, have large thicknesses,while product substrates are always more greatly thinned in order toachieve a correspondingly high packing density of functional units inthe product substrates by stacking. The thickness of a carrier substrateis larger than 200 μm, preferably larger than 500 μm, most preferablylarger than 700 μm, and with utmost preference larger than 1,000 μm. Thethickness of a product substrate is smaller than 1,000 μm, preferablysmaller than 500 more preferably smaller than 100 μm, and with utmostpreference small than 50 μm.

In addition, the alignment unit is equipped with a device for receivingand attaching substrates, which allows it to attach substrates to oneanother after the alignment.

In particular, the alignment unit comprises a device that makes itpossible to align substrates with respect to one another and thus toattach them temporarily with an attaching means so that a transport ispossible from the alignment unit over a vacuum transfer section toanother module, in particular a bonding system, without this transporthaving a negative effect on the aligned substrates. Especiallypreferably, the attaching unit is a device for magnetic attachment ofthe substrate stack, as described in the publication PCT/EP 2013/056620,to which reference is made in this respect. Alternatively, clamping canbe done by mechanical clamps, which clasp the sides of the substrates ina small peripheral section and no longer have to be attached to a bondchuck. The attaching of the substrates below one another by anelectrostatic attractive force between the substrate surfaces would alsobe conceivable. Tacking (sewing) represents another conceivable way ofattaching the substrates to one another. This involves a local bondingor adhesion of the two substrates by the application of a concentratedpoint force, in particular a concentrated electric current or a greatlylimited impact of very high heat, preferably by a laser. Thislocally-limited stressing between the surfaces of the two substratesprovides at least for a local attachment, which is sufficient to be ableto transport the two substrates without causing a shifting of the twosubstrates toward one another.

According to the invention, the individual modules are equipped inparticular with connecting locks that are located in the respectivepassages to the individual modules and with which it is possible toprovide defined atmospheres that have a positive influence on thesubsequent process.

According to the invention, a lock is an area, in particular a space,which is connected by two floodgates from two additional areas, inparticular spaces, separated from one another but connected to thelocks. The access from and/or by the locks in and/or from one of the twospaces is provided by floodgates. The floodgates can be triggered inparticular individually. The floodgates are designed in particular asvalves or gate valves. Hereinafter, no explicit distinction is madebetween floodgates and/or locks. In this respect, it is meant that whenusing the word floodgate, in particular also an entire lock, thereforean area, in particular a space, with two floodgates can be meant.

In the individual modules, it is accordingly possible to producedifferent atmospheres based on the requirements of the respectiveprocess. In addition, it is possible to control processes in the vacuumtransfer module that make possible an acceleration of the entire bondingprocess. In this connection, a heating done upstream from the bondingthat makes it possible to preheat substrates that are to be bonded inorder to deliver the latter, subsequently tempered, to the vacuumchamber module is conceivable. As a result, the time-intensive heatingof the heating systems of the bonding system is significantly shortened,which has a direct effect on the throughput that is to be expected.

In addition, in particular an upstream heating for preconditioning ofthe substrates independently of one another is conceivable. Before theintroduction into the alignment unit, the substrates can preferably beheated to >100° C. In this case, the free path length for molecules onthe surface can be maximized. For example, the substrates can be heatedat a great distance to a chamber wall (>1 cm, >5 cm, >10 cm). The largefree path length that is produced in this case requires that foreignatoms and gases be transported away from the substrate surfaces. Duringtransport of the individual substrates in the alignment unit, thetemperatures preferably fall again to room temperature.

The alignment of the substrates is carried out in the alignment unit inparticular under high-vacuum conditions, which preferably essentiallycorrespond to the pressure conditions in the bonding system. After thesubstrates are aligned, an automated transport is carried out via aprocess robot from the alignment unit to a vacuum transfer module, inwhich preparatory measures for the bonding planned in the subsequentcourse of the process can be carried out.

After the transfer of the substrate stack from the vacuum transfersection module into the bonding module, the substrate stack is laid downby the robot on removable loading pins. A direct contact between theloading pins and the substrate stack is carried out only partially.According to the invention, at most thirty, preferably less than 20,more preferably less than 10, most preferably less than 5, and in mostcases preferably exactly three loading pins, are used.

The substrate stack that is to be bonded thus floats between a dynamicfirst heating surface and pressure plate that can be brought up to thesubstrates and an in particular static, second heating surface. Both thefirst (in particular upper) and the second (in particular lower) heatingsystem and pressure plate advantageously have approximately the samediameter as the substrates to be treated or are larger. The firstheating surface and pressure plate can be brought back up bothsymmetrically and asymmetrically to the substrates. In the case of anasymmetrical heating of the substrate, the separation distance, evenwhen dynamically bringing the first heating surface and pressure plateup to the substrates located in the substrate receiving means, is notequal to the separation distance between the second heating surface andpressure plate of the substrate receiving means that is located betweenthe first and second heating surfaces or to the substrate surfaces thatare to be heated.

The separation distance between the first and/or second heatingsurface(s) and the substrate stack during the loading process is inparticular greater than 1 mm, preferably greater than 5 mm, morepreferably greater than 10 mm, and most preferably greater than 30 mm.

The temperatures of the first and/or second heating surface(s) duringthe loading process are in particular higher than 25° C., preferablyhigher than 100° C., and more preferably higher than 300° C.

The heating rate of the substrate stack is to be controlled inparticular by the separation distances A of the first heating surfaceand/or B of the second heating surface relative to the substrate stackor to the respective surfaces. Thus, it is in particular possibleaccording to the invention to operate temperature programs and/or cycleswithout having to constantly cool or heat the heating systems and thusthe bond chucks and/or the pressure plates. A similar procedure wasalready used in the patent specification PCT/EP2013/064151 in order toheat a liquid by the positioning of a heater without the heater havingto be constantly heated or cooled. Owing to this embodiment according tothe invention, it is possible to heat a heater to a specifictemperature, in particular the bonding temperature, to establish itsposition, however, during the loading process far from the loading planeof the substrate stack so that the loaded substrate stack is initiallyheated to a very small extent (i.e., in particular during loading). Onlyby the approaching is a correspondingly controlled, in particularsymmetrical, heating of the substrates, specified according to theboundary conditions, carried out. This process according to theinvention functions in particular when the heating of the substratestack is carried out at least predominantly, preferably exclusively, byheat radiation, not via heat convection. In particular, this processaccording to the invention has maximum effectiveness under vacuum.

In the case of a symmetrical heating of the substrates, the separationdistance A during a dynamic bringing of the first heating surface andpressure plate up to the substrates is equal to the separation distanceB of the second, in particular static, second heating surface. This isachieved in particular by both controlled and in particular continuousbringing-up of the first heating surface to the second heating surface,whereby at the same time, the receiving system receiving the substratestack is brought close to the second heating surface. The first heatingsurface is preferably moved exactly twice as fast as the substrate stackto the second heating surface.

In an alternative embodiment, the receiving system remains stationarywith the substrate stack, and the two heating surfaces are moved, inparticular at the same speed and under the same symmetrically decreasingseparation distances A and B, from above and below with respect to thesubstrate stack.

For both symmetrical and asymmetrical heating, formulas, determined inparticular empirically in advance or by measurements, are stored in acontrol system and provide for an optimized heating of the substratestaking into consideration existing parameters. As parameters, primarilythe type of material, thickness, heating temperature and bonding methodare considered.

The asymmetrical approaching or positioning primarily has the object ofmaking possible a different heat input into the substrates in order tobe able to compensate for the thermal expansions of two substrates ofdifferent materials. In general, each material has a separate thermalexpansion coefficient. In order to achieve an equal expansion of the twosubstrates, in this case the substrate with the higher thermal expansioncoefficient is heated to a lower temperature than the substrate with thelower thermal expansion coefficient. This can be carried out accordingto the invention by different temperatures on the first heating surfaceand the second heating surface and/or by different separation distancesA and B.

Another aspect according to the invention is that a gas input betweenthe contact surfaces of the substrate stack is prevented after the firstpressure plate and the second pressure plate are brought into contact orthe bond chuck is brought into contact with the substrate stack, andthen a gas is introduced into the bonding system. The latter ispropagated between the rough surface of the substrate stack and the bondchuck or the pressure plates and thermally follows the surfaceunevenness.

Thermal closing is defined as the introduced gas serving as a heattransfer system in order to introduce the heat from the heaters via thebond chuck or the pressure plate as quickly as possible into thesubstrate stack. The gas pressure between the bond chuck and thesubstrate stack is in this case in particular >0.01 mbar,preferably >0.1 mbar, more preferably >1 mbar, and with utmostpreference >3 mbar. This gas pressure is achieved in particular by smallgas amounts (flow rates), preferably of >1 sccm, more preferably >5sccm, and more preferably >20 sccm.

Owing to high pump output, in particular of several hundred liters persecond, at the same time a high vacuum in the bonding chamber, inparticular <0.001 mbar, preferably 0.0001 mbar, and more preferably<0.00001 mbar, can be maintained. In this case, a force acts inparticular on the substrate stack, which prevents a lateral shifting.The applied bonding forces are preferably between 1 N and 200 kN, morepreferably between 1 kN and 100 kN.

From this time, an ordinary bonding process that is already known in thestate of the art is carried out. In this case, this can be a temporarybond or a permanent bond. Temporary bonds with temporary adhesives,permanent bonds such as eutectic bonds, anodic bonds, glass frit bonds,fusion bonds, metal (diffusion) bonds or bonds with permanent adhesiveswould be conceivable.

Although the embodiment according to the invention as well as theprocess according to the invention can be performed with almost all bondchucks, a studded bond chuck is preferred (independent aspect of theinvention). A studded bond chuck is defined as a bond chuck whosesurface is not even but rather has several small protrusions, the studs,which form a holding plane and carry the substrate stack thereon. Thesestuds substantially reduce the contact surface with respect to thesubstrate stack and thus also reduce the probability of contaminatingthe substrate. The studded bond chuck is designed so that it withstandsthe applied bonding force.

The height of the studs is in particular smaller than 1 mm, preferablysmaller than 0.1 mm, more preferably smaller than 0.01 mm, and mostpreferably smaller than 0.001 mm. Especially preferably, the pressureplate also has studs, so that the pressure plate is a studded pressureplate that has the same property according to the invention as thestudded bond chuck.

In another special embodiment, at least one of the heating systems,preferably designed simultaneously as a bond chuck, has a heatingsurface that has several zones. These zones of the heating surface canpreferably be actuated individually. The zones of the heater arepreferably circular rings positioned concentrically to one another.Locally resolved temperature profiles can be produced by the targetedactivation of the zones. The number of zones is greater than 1,preferably greater than 5, more preferably greater than 0.10, mostpreferably greater than 20, most preferably greater than 50, and withutmost preference greater than 100.

All described modules are those in which special atmospheres can beproduced in a reproducible manner based on the material of thesubstrates and the requirements for the respective bonding method. Inaddition, the modules, independently of one another, are adjustable inparticular in an infinitely variable manner. As an alternative, theindividual modules can be operated even without a vacuum, i.e., atnormal pressure. Also, atmospheric synchronization of two adjacentmodules is possible to enable accelerated movement of substrates fromone module into a module that is downstream within the process.

The pressure within the vacuum section and/or within the alignmentsystem and/or within the bonding system is in particular less than 1mbar, preferably less than 10⁻³ mbar, more preferably less than 10⁻⁵mbar, most preferably less than 10⁻⁷ mbar, and with utmost preferenceless than 10⁻⁹ mbar.

Additional advantages, features and details of the invention follow fromthe subsequent description of preferred embodiments as well as based ODthe drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a a diagrammatic overview of a first embodiment of a vacuumcluster with specifically two attached modules.

FIG. 1b a diagrammatic overview of a second embodiment of a vacuumcluster with seven attached modules.

FIG. 2 a diagrammatic cross-section of an embodiment, according to theinvention, of a bonding system before the loading of a substrate stack.

FIG. 3 a diagrammatic cross-section of the embodiment according to FIG.2 when placing the substrate stack on loading pins.

FIG. 4 a diagrammatic cross-section of the embodiment according to FIG.2 in the case of the removal of a robot arm from the substrate stack.

FIG. 5 a diagrammatic cross-section of the embodiment according to FIG.2 in the case of the removal of the robot arm from the module.

FIG. 6 a diagrammatic cross-section of the embodiment according to FIG.2 with symmetrical heating of the substrate with two heating systems.

FIG. 7 a diagrammatic cross-section of the embodiment according to FIG.2 in the case of the symmetrical approaching of the heating systemsrelative to the substrate stack.

FIG. 8 a diagrammatic cross-section of the embodiment according to FIG.2 in the case of the symmetrical approaching of the heating systemrelative to the substrate with contact of the first heating system withparts of the loading pins according to the invention.

FIG. 9 a diagrammatic cross-section of the embodiment according to FIG.2 when the substrate stack is brought into contact symmetrically.

FIG. 10 a side view of a cross-section of the surrounding area of astudded bond chuck and a studded pressure plate.

FIG. 11 a top view of a studded bond chuck according to the invention,and

FIG. 12 a top view of a studded pressure plate according to theinvention.

In the figures, the features that are the same or that have the sameeffect are identified with the same reference numbers.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1a shows a diagrammatic overview of a unit 38 that is designed inparticular as a vacuum cluster, preferably as a high-vacuum cluster. Theunit 38 is comprised of precisely two modules attached to a vacuumtransfer chamber 4, a module with an alignment system 1 and a modulewith a bonding system 6 according to the invention. A robot 34 that isdesigned in particular as a process robot draws substrates 35, 36(identical here) from a loading container 39 and transports the firstsubstrate 35 and the second substrate 36, in particular at the sametime, along a vacuum transfer section 5 into the alignment unit 1. Theloading container 39 can also in particular be a lock or can act assuch. The two substrates 35, 36 are aligned with one another and areattached, in particular temporarily, on a first contact surface 35 k ofthe first substrate 35 and a second contact surface 36 k of the secondsubstrate 36 to form a substrate stack 14. As an alignment unit, forexample, the units from the patent specifications PCT/EP 2013/075831 orPCT/EP 2013/062473 could be used. An optimal alignment is then providedwhen the structures that are to be aligned with one another, inparticular also the alignment marks, optimally fit into one anotheraccording to the overlay model known in the industry. A correspondingdescription of such an overlay model is found in the patentspecification PCT/EP 2013/061086.

Then, the robot 34 transports the attached and aligned substrate stack14 in the bonding system 6, in particular by receiving the firstsubstrate 35 on a first surface 35 o or the second substrate 36 on asecond surface 36 o. The surfaces 35 o, 36 o are in each case facingaway from the contact surfaces 35 k, 36 k.

The unit 38′ according to FIG. 1b shows a vacuum cluster that iscomprised of several modules connected by a vacuum transfer chamber 4′.The modules may differ from one another in their functionality. Inparticular, modules for-heating or cooling substrates or substratestacks, purification modules, plasma modules, enameling modules withcentrifugal enameling devices or spray-enameling devices, bonders 1 anddebonders, coating modules, and alignment modules 6 are conceivable. Themodules are preferably arranged in a circular or star-shaped manneraround a vacuum transfer chamber 4′.

The vacuum transfer chamber 4′ is connected via valves 2 to the variousmodules. The modules as well as the vacuum transfer chamber 4′ can beevacuated independently of one another by the valves 2, but are alwayspreferably located on the same vacuum preferably the high-vacuum levelof the bonding system 6.

The bonding system 6 is depicted in FIGS. 2 to 9 in various processingstates. The bonding system 6 is designed on a static supportingstructure 23 in the form of a base plate and on columns attached to thebase plate. The bonding chamber 10 is attached to the columns.

The bonding chamber 10 has a chamber opening 6 o that can lock with thevalve 2 for loading the bonding chamber 10.

The valve 2 is formed from a lock drive 24, in particular in the form ofan actuator, supported on the base plate. The lock drive 24 serves toopen and close a floodgate 27 that is driven by the lock drive 24, afloodgate that opens and closes the chamber opening 6 o by a slot 6 s.The valve 2 has seals 28 for sealing the bonding chamber 10 against thesurrounding area in the closed state of the valve 2.

In addition, the bonding system 6 comprises a receiving system 18 forreceiving the substrate stack 14. The receiving system 18 comprises asubstrate base with abase plane E, on which the substrate stack 14 islaid down with the second surface 36 o, so that the second surface 36 olies in the base plane E.

The substrate base is formed by at least two loading pins 21, in theembodiment shown four loading pins 21, running through the bondingchamber 10. The bonding chamber 10 is sealed by seals 20 surrounding theloading pins 21 relative to the surrounding area of the bonding chamber10. The seals 20 simultaneously serve for sliding, translatory guidingof the loading pins 21.

The latter are attached on an in particular common adjustment plate 21 pon the ends of the loading pins 21 opposite the substrate base. Theloading pins 21 are preferably coupled to one another mechanically bythe adjustment plate 21 p and are moved in a translatory mannercrosswise to the base plane E by an adjustment drive 22 that acts inparticular centrically on the adjustment plate 21 p, an adjustment drive22 preferably in the form of a single loading pin actuator, or as analternative by means of several loading pin actuators.

Within the loading pins 21, a second heating system 26 is arranged forheating the second surface 36 o and an in particular full-surface,attached second pressure plate 25 on the second heating system 26. Thesecond pressure plate 25 has a second heating surface 19, which can bearranged below the base plane E, parallel to the latter. The heatingsystem 26 and the pressure plate 25 are connected securely to thebonding chamber 10 and are static, i.e., cannot move relative to thebase plane E.

Opposite to the second heating surface 19, a first heating surface 15can be arranged parallel to the base plane E and above the latter. Thefirst heating surface 15 is arranged on a first pressure plate 29, whichin turn is attached to a first heating system 30, in particular on thefull surface.

The heating system 30 can be adjusted by drive means crosswise to thebase plane. The heating system 30 is attached to an adjustment rod thatruns through the bonding chamber 10. The adjustment rod is moved on theend opposite to the heating system 30 from a positional actuator 8 tocontrol the position, in particular a separation distance A from thefirst heating surface 15 to the first surface 35 o of the first heatingsurface 15. For pressurization, in particular after bringing the firstsurface 35 o into contact with the first heating surface 15 and bringingthe second surface 36 o into contact with the second heating surface 19,a force actuator 9, which can apply the higher compressive force that isnecessary for bonding, is used. The bonding chamber 10 is sealed by theseals 31, sealing the drive means, relative to the surrounding area.

The drive means are suspended on a supporting structure 7, comprising ofa cover plate and columns supporting the cover plate.

The process according to the invention is described below based on FIGS.2 to 9.

In a first step according to the invention in accordance with FIG. 2,the robot 34 is moved, in particular transferred, with the substratestack 14 into a bonding chamber 10 of the bonding system 6. In the firststep, the receiving system 18 according to the invention is located on astarting level for receiving the substrate stack 14. In the startinglevel, the substrate stack 14 relative to the contact surfaces 35 k, 36k is preferably positioned symmetrically to the first heating surface 15and to the second heating surface 29. This symmetrical starting positionis primarily important when the second pressure plate 25 and/or thefirst pressure plate 29 were preheated by the corresponding heatingsystems 26, 30 thereof.

In a second step according to the invention in accordance with FIG. 3,the positioning of the substrate stack 14 is done in such a positionthat the attachments ii for attaching the substrate stack, in particularmagnetic clamps 11, with the heating surfaces 15, 19 subsequently beingbrought together, can be received precisely in recesses of the pressureplates 25, 29 provided for this purpose.

In addition, attention must be paid that the substrate stack 14 isloaded and positioned as centrically as possible to the loading pins 21in order to prevent sliding or slipping.

In a third step according to the invention in accordance with FIG. 4,the robot 34 is removed from the substrate stack 14 so that thesubstrate stack 14 rests on the loading pins 21. The second top side 36o now lies in the base plane E.

In a fourth step according to the invention in accordance with FIG. 5,the removal of the robot 34 from the bonding chamber 10 as well as theclosing of the lock 27 are carried out. After the closing, the interiorof the bonding system 6 can be evacuated via a pump 16 with a stillhigher vacuum should the vacuum prevailing in the adjoining vacuumtransfer chamber 4 be set too low for the bonding process.

In a fifth step according to the invention in accordance with FIG. 6,the symmetrical heating of the two surfaces 35 o, 36 o of the substratestack 14 facing away from one another is now carried out. It would alsobe conceivable, of course, that the two heaters 26 and 30 were alreadyset and were kept at bonding temperature before the insertion of thesubstrate stack 14, which reduces the heating time of the pressureplates 29, 30 and the heating systems 26, 30 virtually to zero.

The heating is done by heat output produced by the heating systems 26,30 and released over the heating surfaces 15, 19 as radiation heat 17.

The idea according to the invention is shown in particular in thisprocess step. By the symmetrical positioning of the substrate stack 14,the temperature fields above and below the substrate stack 14 can be setin a fully equivalent manner, provided that the two heating systems 26,30 are controlled with the same output and the same parameters and thepressure plates 25, 29 have the same or at least very similar propertiesand geometries/dimensions.

The substrate stack 14 is in particular not limited over the entiresurface by a frictional force in its radial thermal expansion, butrather rests only peripherally on the loading pins 21. As a result, itcan expand almost free-floating symmetrically, without stresses orbulges being caused.

A further significant advantage is that the contact of the twosubstrates 35, 36 of the substrate stack 14 with two heating surfaces15, 19 is avoided at least during the heating process.

In a sixth step according to the invention in accordance with FIG. 7,the symmetrical approaching of the substrate stack 14 onto the secondheating surface 19 and the first heating surface 15 is carried out. Thesecond pressure plate 25 (in particular bond chuck) is in this casestatic and does not move. Rather, the loading pins 21 are moved by meansof the loading pin actuators 22 onto the first heating surface 15. Atthe same time, the first heating surface 15 is moved to the secondheating surface 19 or the substrate stack 14.

In order to keep the separation distance A between the substrate stack14 and the first heating surface 15 equal to the separation distance Bbetween the substrate stack 14 and the second heating surface 19, thefirst heating surface is moved at twice the speed of the loading pins21. Another speed profile would also be conceivable, however, in orderto produce a specific separation function and thus temperature functionon the substrate stack and thus an at least partially asymmetricalapproaching.

According to the invention, a reversal in which the first heatingsurface 15 is statically designed and the loading pins 21 as well as thesecond heating surface 19 move in the direction of the first heatingsurface 15 would also be conceivable.

In a special embodiment, an in particular identically quick, reversemovement of the two heating surfaces 15, 19 in the case of staticloading pins 21 would also be conceivable.

In a seventh step according to the invention in accordance with FIG. 8,finally the heating surfaces 15, 19 are brought into contact with thesubstrate stack 14. The force that is applied in this connection by thepositional actuator 8 is sufficient to press both substrates 35 and 36on one another so strongly that the thus produced frictional force nolonger allows a mutual shifting along the base plane E (frictionalconnection).

The bonding system 6 can now be flushed with gas.

Preferably, the gas is introduced by lines within the second pressureplate 25 and/or first pressure plate 29 and is distributed when using afirst pressure plate 29 and/or second pressure plate 30 with studs 37 oran additional studded pressure plate 42, 42′, each applied on thepressure plates, between the studs 37 in the flow channels 32.

Gas that is fed between the studs 37 can be held up by an arm 40 that islocated on the studs 37 on the sides of the pressure plate 25, 29 andthat extends over the entire radial side area. The arm seals inparticular in a positive manner with the substrate 35, 36 to beprocessed on its peripheral side.

In addition, it is conceivable to provide the arm 40 with a passage 41that makes possible a controlled leakage of the excess gas from thestudded surface. The passage 41 is advantageously smaller than 10 μm indiameter, preferably smaller than 7 μm, and more preferably smaller than5 μm.

If no studs 37 or studded pressure plates 42, 42′ are used, the gas isdistributed by the existing surface roughness of the heating surfaces15, 19, which replaces the studs 37.

FIG. 12 shows a diagrammatic top view, not to scale, of the studdedpressure plate 42, attached to the first pressure plate 29, with severalstuds 37 with a stud height H. The density of the studs 37 is kept verylow in FIGS. 11 and 12 in order to increase clarity.

Preferably, the studded pressure plates 42, 42′ in each case have atleast 50, in particular regularly- and/or equally-distributed, studs 37,more preferably in each case at least 100, more preferably in each caseat least 200, and more preferably in each case at least 400.

FIG. 11 shows a diagrammatic sectional view according to the line ofintersection A-A of FIG. 10 with the second pressure plate 25 with thestudded pressure plate 42′. In this depiction, it is recognized that theentire surface of the substrates 35, 36 does not rest on the studs 37,thereby the probability of the contamination of the substrates 35, 36 isreduced.

The separation distances A and B are reduced to zero, so that contactexists between heating surfaces 15, 19 of the studded pressure plates42, 42′ and the surfaces 35 o, 36 o.

LIST OF REFERENCE SYMBOLS

-   1 Alignment system-   2 Valves-   3 Sealing ring-   4, 4′ Vacuum transfer chamber-   5 Vacuum transfer section-   6 Bonding system-   6 o Chamber opening-   6 s Slot-   7 Supporting structure-   8 Positional actuator-   9 Force actuator-   10 Bonding chamber, in particular vacuum chamber-   11 Attachments-   14 Substrate stack-   15, 15′ First heating surface-   16 Pump-   17 Radiation heat-   18 Receiving system, in particular substrate base-   19, 19′ Second heating surface-   20 Seals-   21 Loading pins-   21 p Adjustment plate-   22 Adjustment drive, in particular loading pin actuator-   23 Supporting structure-   24 Lock drive-   25 Second pressure plate-   26 Second, in particular lower, heating system-   27 Valve-   28 Lock seals-   29 First pressure plate-   30 First, in particular upper, heating system-   31 Seals-   32 Flow channels-   34 Robot, in particular process robot-   35 First, in particular upper, substrate-   35 k First contact surface-   35 o First surface-   36 Second, in particular lower, substrate-   36 k Second contact surface-   36 o Second surface-   37 Studs-   38, 38′ Unit, in particular vacuum cluster, preferably high-vacuum    cluster-   39 Locks-   40 Arm-   41 Passage-   42, 42′ Studded pressure plate-   A Separation distance-   B Separation distance-   E Base plane-   H Stud height

Having described the invention, the following is claimed:
 1. A devicefor bonding a first substrate to a second substrate, the devicecomprising: a first heating system having a first heating surface thatprovides heat by infrared radiation; a second heating system having asecond heating surface that provides heat by infrared radiation; areceiving system for receiving a substrate stack comprised of (i) afirst substrate having a first contact surface and a first surfacefacing away from the first contact surface and (ii) a second substratehaving a second contact surface and a second surface facing away fromthe second contact surface, the first and second substrates aligned onthe first and second contact surfaces, wherein the receiving system isarranged between the first heating surface of the first heating systemand the second heating surface of the second heating system, the firstand second substrates respectively spaced from the first and secondheating surfaces by separation distances A and B, the first heatingsurface faces the first surface of the first substrate, and the secondheating surface faces the second surface of the second substrate, drivemeans for changing the separation distances A and B; a force actuatorfor applying pressure to the first and second surfaces of the substratestack to form a bond between the first contact surface of the firstsubstrate and the second contact surface of the second substrate; and acontrol system for controlling operation of the first heating system,the second heating system, the drive means, and the force actuator, saidcontrol system operable in (i) a heating mode to heat the first andsecond substrates, wherein the control system controls the drive meansto maintain the separation distances A and B such that A>0 μm and B>0μm, and independently regulate the first and second heating systems inaccordance with parameter data to heat the first and second substratessuch that a thermal expansion of the first substrate and a thermalexpansion of the second substrate are substantially the same during theheating mode, and (ii) a bonding mode to form a permanent bond betweenthe first and second substrates by pressurization of the substratestack, wherein the control system controls the drive means to approachthe substrate stack towards the first and second heating surfaces byreducing the separation distances A and B to 0 μm, and controls theforce actuator to apply pressure to the first and second surfaces of thesubstrate stack to form the bond between the first contact surface ofthe first substrate and the second contact surface of the secondsubstrate.
 2. The device according to claim 1, wherein the first heatingsurface and/or the second heating surface are formed by studs with flowchannels running between the studs for pressurization with a gas.
 3. Thedevice according to claim 2, wherein the flow channels are surrounded byan arm for sealing the flow channels with respect to the first and/orsecond substrate.
 4. The device according to claim 3, wherein the armhas an opening for controlled discharge of gas.
 5. The device accordingto claim 3, wherein the arm is a circular arm.
 6. The device accordingto claim 2, wherein the flow channels are flushed with gas.
 7. Thedevice according to claim 1, wherein, the control system independentlyregulates the first and second heating systems in accordance with theparameter data by (i) heating the first heating surface to a differenttemperature than the second heating surface and/or (ii) maintainingdifferent separation distances A and B.
 8. The device according to claim1, wherein the parameter data is indicative of: type of material of thefirst and second substrates, thermal expansion coefficients of the firstand second substrates, thickness of the first and second substrates,heating temperature of the first and second heating surfaces, and/orbonding method.
 9. A method for bonding a first substrate to a secondsubstrate, the method comprising: providing a substrate stack comprisedof (i) a first substrate having a first contact surface and a firstsurface facing away from the first contact surface and (ii) a secondsubstrate having a second contact surface and a second surface facingaway from the second contact surface, said substrate stack aligned onthe first and second contact surfaces; arranging the substrate stackbetween a first heating surface of a first heating system and a secondheating surface of a second heating system by use of a receiving system,wherein the first heating surface faces the first surface of the firstsubstrate, and the second heating surface faces the second surface ofthe second substrate, the first and second substrates respectivelyspaced from the first and second heating surfaces by separationdistances A and B; heating the first and second substrates in a heatingmode wherein the separation distances A and B are maintained such thatA>0 μm and B>0 μm, said heating mode including independent regulation ofthe first and second heating systems in accordance with parameter datato heat the first and second substrates such that a thermal expansion ofthe first substrate and a thermal expansion of the second substrate aresubstantially the same during the heating mode; and bonding the firstand second substrates in a bonding mode to form a permanent bond betweenthe first and second substrates by pressurization of the substratestack, said bonding mode including: approaching the substrate stacktowards the first and second heating surfaces by use of drive means toreduce the separation distances A and B to 0 μm, and applying pressureto the first and second surfaces of the substrate stack by use of aforce actuator to form the bond between the first contact surface of thefirst substrate and the second contact surface of the second substrate.10. The method according to claim 9, wherein the method furthercomprises: preheating the substrate stack before arranging the substratestack between the first and second heating surfaces.
 11. The methodaccording to claim 10, wherein the method further comprises preheatingthe substrate stack outside a bonding chamber of a bonding system. 12.The method according to claim 11, wherein the bonding system surrounds,vacuumably, the first and second heating systems.
 13. The methodaccording to claim 9, wherein the approaching of the substrate stacktowards the first and second heating surfaces is carried outsymmetrically with respect to the substrate stack.
 14. The methodaccording to claim 13, wherein the approaching of the substrate stacktowards the first and second heating surfaces is carried outsymmetrically to the respective first and second surfaces of the firstand second substrates and/or the respective first and second contactsurfaces of the first and second substrates and/or at the sametemperature of the first and second heating surfaces.
 15. The methodaccording to claim 9, wherein the first and second heating surfaces arelarger than the respective first and second surfaces of the substratestack that respectively face the first and second heating surfaces. 16.The method according to claim 9, wherein the first heating surface orthe second heating surface is not moved during the approaching.
 17. Themethod according to claim 9, wherein the method further comprises:controlling the first and second heating systems during the heating andthe approaching such that at least during a substantial majority of atime period for approaching, the difference between the mean temperatureof the first substrate and the mean temperature of the second substrateis less than 5° C.
 18. The method according to claim 9, wherein thearranging of the substrate stack between the first and second heatingsurfaces and/or the heating and/or the approaching and/or the bondingis/are carried out in a vacuum.
 19. The method according to claim 9,wherein said method further comprises: controlling the first and secondheating systems during the heating and the approaching such thatradiation energy of the first heating surface striking the first surfaceof the first substrate during the approaching is the same as theradiation energy of the second heating surface striking the secondsurface of the second substrate.
 20. The method according to claim 9,wherein the first and second heating systems are independently regulatedin accordance with the parameter data by (i) heating the first heatingsurface to a different temperature than the second heating surfaceand/or (ii) maintaining different separation distances A and B.
 21. Themethod according to claim 9, wherein the parameter data is indicativeof: type of material of the first and second substrates, thermalexpansion coefficients of the first and second substrates, thickness ofthe first and second substrates, heating temperature of the first andsecond heating surfaces, and/or bonding method.